A volume holographic storage device writes and reads data in an optical form. See, for example, S. Redfield and L. Hesselink, "Data Storage in Photorefractives Revisited," Optical Computing 88, Proc. SPIE, Vol. 963, Toulon, France, pp. 35-45 (1988), which reports investigations of a holographic data storage technology (hereinafter Redfield). The technology of Redfield encodes the information to be stored in an optical recording medium as a two-dimensional array of spots called a "page." The optical recording medium for a write/read subsystem may be a photorefractive crystal. Pages are stored as patterns in the distribution of donor electrons of the photorefractive crystal which locally affect the index of refraction of the material. The pages are actually stored as a hologram of the Fourier transform of the spot array. The Fourier transform is a convenient way to obtain a small recording size and provides some immunity to crystal material defects. Pages are placed in the photorefractive crystal as a two-dimensional array of stacks of pages.
By storing the pages in stacks and having an array of these stacks, a holographic storage system achieves three-dimensional use of the recording media. Another advantage of using pages of holographically stored information is that read access times may increase to be 100 to 1,000 times faster than those obtainable with magnetic or optical disks. This is because readout of information can be done by simply directing light beams. This may be done with no moving parts. Further, this accessing involves reading out images, instead of single bits serially as is the case with magnetic recording. The result is a significant improvement in bandwidth. The holographic storage technology leverages the imaging properties of light and the ability of light to be launched.
While holographic storage technology has been investigated in the past, its use has not been generally accepted. One significant reason has been the immature state of the art of related technologies such as two-dimensional spatial light modulators (SLMs) or page composers, laser devices, beam deflectors, optical recording materials, and detector arrays. While considerable advances in recent years in all of these components make volume holographic optical storage an attractive technology, as Redfield explains, a significant limitation still remains in this technology with regard to the size and complexity of the optical configuration necessary to holographically store a large amount of data in an optical storage medium.
The example of a holographic storage system that Redfield describes includes an object path along which light travels from a laser through a shutter and on to an X-Y deflector system and a beam splitter. From the beam splitter, the light first encounters a device called a flys eye which corresponds to an array of small lenses, one for each stack. Next, the light passes through a transform lens and SLM. The transform lens is placed immediately in front of the SLM. It focuses through the SLM onto the region in the optical storage medium, where a page is to be written, and forms the Fourier transform of the SLM pattern at that place in the holographic storage medium. The optical storage medium is mounted to a holder which has a thermoelectric cooler/heater at its base for heating and cooling the storage medium. On the output side of the crystal is an imaging lens which images the output pattern onto the detector array. The detector array then performs high speed optical-to-electrical conversion of the data. The internal sequencing of the Redfield device may be accomplished by a microprocessor which controls exposures, loads the SLM, deflects beams, sequences reads and writes, and shifts data off the detector array.
The configuration of Redfield requires a multitude of optics devices for creating the necessary interference pattern from the reference beam and object beam, the movement of any one of which could adversely affect the overall result. Moreover, the Redfield configuration requires an optical path that covers typically 14 optical diameters. This inherent complexity and the associated size requirements yield a number of limitations. For example, this system demands a long narrow form factor which does not conveniently fit in a computer chassis. In addition, each of the optical elements requires individual placement. This placement requirement makes high manufacturing costs for a such system unacceptably high. The configuration of Redfield also requires an additional beam deflector for the reference beam. This adds further to the expense of the device.
In Redfield, as is the case with other holographic recording configurations, the device holds the read/write media fixed while reference and object beams are positioned to angle multiplex pages at different stack sites in the medium. The reference beam and object beam are first moved to a stack site in tandem, then an angle is placed on the reference beam. This means there are a very large number of possible beam positions. This large number of possible beam positions makes reading and writing data extremely complex. These systems also possess a limited recording medium window, i.e., the area on the optical medium on which data may be recorded without moving the medium.
Consequently, there is a need for an improved holographic storage system that does not require long distance or large spaces to write and read holographic images.
There is a need for a holographic storage system that avoids the multitude of individual optical components of conventional holographic storage systems and is economical to manufacture.
There is a further need for a holographic storage system that has simplified beam positioning and that limits multiple page and multiple stack beam positioning to one dimension and/or the use of phase encoding to eliminate reference beam angle deflection.
There is yet a further need for a holographic recording system that permits the use of a common deflector for both the reference and object beams so as to further minimize the cost of system manufacture, operation, and repair.